Interseismic Coupling in the Central Nepalese Himalaya: Spatial Correlation with the 2015 Mw 7.9 Gorkha Earthquake

Geodetic measurements conducted in the Himalaya over the last two decades have shown that the shallow portion of the main himalayan thrust (MHT) was entirely locked during the interseismic period. The induced elastic strain accumulated on the MHT beneath the Lesser Himalaya was not released until the 2015 Gorkha Mw 7.9 earthquake, which ruptured the north edge of the locked portion of the MHT. We utilized our own Global Positioning System (GPS) data from southern Tibet, combined with published geodetic velocities, to quantify the spatial variations of the coupling that prevailed before the Gorkha earthquake. The refined coupling model shows that the MHT was strongly locked (coupling > 0.5) in the uppermost 15 km of crust, corresponding to a downdip width of ~ 100 km. This model suggests a sharp transition zone of strain accumulation, with a rapid decrease in the coupling coefficient from 1.0 to less than 0.2 along ~ 50 km of the MHT, coinciding with the locations of microseismicity. We also determined slip models for the 2015 Gorkha earthquake and its Mw 7.3 aftershock, considering the ramp–flat–ramp–flat structure of the MHT. We found that ~ 85% of the total moment released by the Gorkha earthquake was concentrated on the partially coupled transition portion of the MHT, indicating that the earthquake mainly ruptured the brittle/ductile transition zone. The coseismic Coulomb failure stress increased along the southern and western parts adjacent to the rupture zone, pushing these two regions closer to failure. The moment deficits that have accumulated in these regions could trigger Mw 8.0 and Mw 8.3 earthquakes, respectively.